The following disclosure includes (minimally edited) the paper by Thomas J. Kelly, Russel H. Barnes, and William A. McClenny, Real-time Monitors for Characterization of Formaldehyde in Ambient and Indoor Air, Proceedings of the 1989 EPA/A&WMA International Symposium, Measurement of Toxic and Related Air Pollutants; Raleigh, N.C., May, 1989, pp. 43-50. Air & Waste Management Association Publication VIP-13; EPA Report No. 600/9-89-060. [Portions enclosed in brackets do not relate directly to the present claimed invention.]
In this paper, the design considerations for two types of prototype gaseous formaldehyde monitors are reviewed and the experimental results of preliminary testing are discussed. The prototypes are designed to measure single digit ppbv to sub-ppbv (parts per billion volume) concentration levels in indoor and ambient outdoor environments. [One unit uses gas phase fluorescence for formaldehyde detection. For this approach, a commercially available monitor, originally designed for detection of sulfur dioxide, was altered by replacement of optical components and adjustment of other design features. Theoretical considerations indicate that this approach should be successful in achieving the desired level of detection sensitivity and selectivity for formaldehyde.] A second prototype was based on an improved design of a system using a wet scrubber for selective sampling of formaldehyde followed by an analytical procedure using the Hantzsch method; that is, cyclization of a .beta.-diketone, an amine, and formaldehyde to form a dihydropyridine derivative which can be detected by fluorescence. Design options were considered including the choice of excitation wavelength and optical components for fluorescence detection.
Formaldehyde (HCHO) is the most abundant aldehyde in the ambient atmosphere, originating both from primary emissions in combustion sources and from atmospheric oxidation of hydrocarbons. Formaldehyde produces free radicals upon photolysis, contributing to the formation of ozone and other oxidants. Concentrations of formaldehyde in the ambient atmosphere range from below 1 ppbv in rural areas to several tens of ppbv in urban areas such as the Los Angeles basin. [1-5] A pronounced diurnal variation is observed in Los Angeles [2] due to the impact of both local sources and photochemistry, and a pronounced seasonal variation is observed in rural areas [3] due to seasonal changes in photochemical activity. Formaldehyde is also found in indoor air, originating from a variety of products. A national database on concentrations of volatile organic compounds [4] indicates that indoor formaldehyde concentrations are typically several times higher than outdoor concentrations. In either indoor or outdoor air, the presence of formaldehyde is important because of the toxicity of this chemical, including suspected carcinogenesis in humans.
Because of the importance of gaseous formaldehyde from both an atmospheric chemistry and a toxicology viewpoint, several methods have been developed for measurement of formaldehyde in air, and intercomparisons of methods have recently been performed. [2,5] Spectroscopic methods include Fourier transform infrared absorption (FTIR), differential optical absorption spectroscopy (DOAS), and tunable diode laser absorption spectroscopy (TDLAS). All are capable of real-time HCHO measurement, of importance in studying the short-term variations in ambient HCHO which convey information about its sources and sinks. However, all three spectroscopic devices are large, complex, and expensive, and only the TDLAS method appears to have sensitivity adequate for measurement of HCHO at the sub-ppbv levels characteristic of rural air. Smaller and less complex real-time HCHO detectors have also been developed, based on continuous collection of HCHO in aqueous solution for subsequent analysis by colorimetry, [e.g., 6] fluorescence, [7,8] or enzyme-catalyzed fluorescence. [9] These methods can provide high sensitivity, but they are subject to some operational difficulties. [2,10,11] Integrated collection and derivatization of HCHO with 2,4-dinitrophenyl-hydrazine [e.g., 3,11,12] can also provide high sensitivity but is not amenable to real-time analysis.
The purpose of the study was to develop new, sensitive, portable methods for real-time measurement of formaldehyde in air. Two methods have been developed to the prototype stage, one an improved wet scrubbing/fluorescence device, [the other a novel spectroscopic approach.]
After a survey of existing collection devices for gaseous HCHO, and a review of analytical approaches for HCHO in the aqueous phase, it appeared feasible to develop an improved HCHO monitor based on an aqueous scrubber with subsequent fluorescence analysis by the Hantzsch reaction, the cyclization of a .beta.-diketone, an amine, and formaldehyde. [13] This analytical approach has been used previously, [7,8] employing a diffusion scrubber tube as the collection device for gaseous HCHO. Although the large air-to-water contact ratio provided by the scrubber allows sub-ppbv detection limits for gaseous HCHO, [7,8] the method suffers from the difficulty of assembling the diffusion scrubber and the very limited lifetime of the scrubber in continuous use. [2] However, we considered that improved analytical sensitivity for HCHO in the aqueous phase might allow use of a simpler and more reliable collection device, such as the glass coil used in another method for HCHO. [9] It appeared that such improvement in sensitivity might be possible by using the high intensity of the 254 nm Hg line for excitation of the fluorophore, [13] rather than the 410 nm excitation commonly used. [7,8] Thus the development of the wet chemical HCHO monitor followed the hypothesis that 254 nm excitation might provide improved sensitivity, thereby allowing use of a simpler and much more reliable collection device for gaseous HCHO.
The wet chemical HCHO monitor has been built around a Turner 112 fluorometer with a quartz HPLC fluorescence flow cell, and a Gilson Minipuls 2 peristaltic pump for circulation of scrubbing and reagent solutions. A glass 28-turn Autoanalyzer coil is used as the collector for gaseous HCHO, contacting a flow of 2 L/min air with 0.8 ml/min of 0.1N H.sub.2 SO.sub.4 as scrubber solution. Reagent concentrations (ammonium acetate/acetic acid buffer, 2,4-pentanedione) are similar to those reported previously. [7] Standard UV lamps (G4 type) were compared as the excitation source to G4 and black-light lamps coated with a phosphor designed to emit maximally at 406 nm (BHK, Inc.); lamps coated with this phosphor were used in previous studies using this analysis method. [7,8]
[The spectroscopic method is based on gas-phase fluorescence detection of HCHO using UV excitation, an approach currently used to measure gaseous SO.sub.2 in commercial devices. A review of literature in the area and extensive feasibility calculations indicated that direct gas-phase fluorescence should be a suitable method of HCHO detection. For practical reasons it was decided to assemble a prototype HCHO detector by modifications to a commercial fluorescence SO.sub.2 detector. The commercial detector chosen to serve as the basis for the modifications is a Thermo Environmental Model 43-S; this detector was chosen because of its high sensitivity (detection limit for SO.sub.2 about 0.1 ppbv) and excellent performance in field measurement programs. Feasibility calculations addressing the 43-S instrument indicated that HCHO detection could be achieved by three primary modifications to the 43-S: (1) changing excitation and emission filters to match the appropriate wavelengths for HCHO, (2) increasing the power output of the pulsed UV lamp, and (3) increasing the gain of the amplifier electronics. The first and third modifications have been performed on a 43-S instrument to produce our initial prototype. A gas-phase HCHO source based on decomposition of trioxane [14] has been assembled and used in initial testing of the prototype, as described below.